Why are people so greedy?

Do you ever wonder what it would be like if you didn't help anyone but yourself?
I think anyone can look out for Number One. I think so as long as they don’t harm anyone one else. For Example, I don’t think the guy from the parlor should’ve taken Scrooge’s things because that is disrespectful and stealing. Even though Scrooge was a horrible man I don’t think anyone deserves to be robbed of their belongings while they are dead. I think everyone should be respected equally after life (unless they are royalty or killers of any form). Again, I think “looking out for Number One” can be the basis of one’s decisions as long as the person’s decision’s doesn’t harm anyone.
So, I ask again, why are people so greedy?